In continuous casting the attainment of high casting speeds and hence an ever greater productivity, while both the surface quality and the inner quality of the product is maintained high, is correlated to the optimisation of a plurality of technological parameters concerning the characteristics of the crystalliser and the equipment connected thereto, and also the casting procedure.
These parameters concern the geometrical and dimensional characteristics of the crystalliser, the cooling system, the system to lubricate the inner walls and the processes to which the product being formed is subjected.
In conventional continuous casting plants, the problems connected with the high temperatures reached by the walls of the crystalliser condition the choice of the parameters decisively, considerably limiting the casting speeds which can be obtained because of the deformations of the crystalliser and the reduction of the mechanical properties of the copper at high temperatures.
To be more exact, the lack of uniformity in temperature along the walls of the crystalliser causes a non-uniform deformation of the walls, due to the thermal expansion of the material, and therefore problems concerned with the surface defects which this deformation causes on the product being formed.
Moreover, as it goes down the crystalliser, the skin of the product being formed tends to shrink as it solidifies.
This causes the skin to become detached from the wall of the crystalliser, which enormously reduces the heat exchange between the product and the crystalliser, to such an extent that the cooling, and thus the solidification of the skin, is practically blocked, which causes the skin to re-melt.
Cracks are created in the skin which, when the cast product is removed, may propagate and cause the product to break, and consequently the liquid metal inside the product can leak out (the break-out phenomenon).
In the case of a product with a square, rectangular or generally polygonal section, another problem is that the corners are subjected to a more intense cooling since they are subjected to simultaneous cooling on several sides of the crystalliser.
In correspondence with the corners, the skin forms more quickly and the consequent shrinkage of the material causes the product being formed to become detached from the wall of the crystalliser very quickly, thus interrupting the cooling and solidification process, and therefore making the temperature of the solidified part increase drastically.
For this reason, in correspondence with the corners, the skin of the product being formed has a lesser thickness than on its plane faces and differences of temperature are created between the edges and the plane faces of the product.
These differences generate tensions which lead to the formation of cracks and other surface defects, which lower the quality of the product and can also cause the skin to break and the liquid steel to break out.
In continuous casting plants used at present, it has been impossible to find a satisfactory solution to all these problems, and furthermore the attempt to solve some of them has led to an accentuation of others.
So, for example, the attempt to increase the casting speed has led to an unsatisfactory cooling of the product being formed, and therefore the solidification of an insufficient thickness of skin, with consequent problems in the removal and pre-rolling of the product emerging from the crystalliser.
On the other hand, any attempt to obtain an optimum cooling of the product has led to a reduction in the casting speed and therefore a reduction in productivity.
Moreover, adapting the conformation of the crystalliser to the shrinkage of the skin of the product being formed in every longitudinal zone of the crystalliser, with the purpose of guaranteeing maximum efficiency of heat exchange, has led to problems of friction between the walls of the crystalliser and the product being formed, and therefore the lowering of the surface quality of the product.
The present Applicant has devised, tested and embodied this invention to overcome the shortcomings of the state of the art and to obtain further advantages, such as in particular a tangible increase in the casting speed.